1. Field of the Invention
The present invention relates to motor vehicle cabin climate control and more particularly to a system for mixing outside and recirculated inside air for secondary climate control zones in vehicles having multiple zones for heating, ventilation and air conditioning (HVAC) control.
2. Description of the Problem
Motor vehicles having relatively large passenger cabins, such as extended cab trucks and recreational vehicles, often provide zonal heating and cooling for different sections of the cabin. Separating heating and cooling functions by zone allows heating and cooling to be limited to those zones that are in use, reducing the load on the heating and cooling plants, particularly the compressor for air conditioning. Typically the zones are arranged from front to back of the vehicle. Each zone will have an independent HVAC unit, located in its own merged heating and cooling air circulation system to reduce the quantity and cost of duct work. HVAC units comprise a blower, an evaporator and a heater core. One air conditioning compressor typically serves all of the evaporators.
The forward operator area is typically provided with an HVAC unit which has vent controls allowing air to be drawn from within the cabin (recirculation air), which can be heated or cooled more quickly, or allowing air to be drawn from outside the vehicle to freshen the air within the cabin. Rapid cooling of a zone is obtained by recirculating cabin air rather than drawing in outside air. When freshening of the air in the operator area is desired, outside air can usually be conveniently drawn through a grill placed just ahead of and under the front windshield of the vehicle. Zones located toward the rear of the cabin have typically drawn only inside air, in part due to the concern for the load being place on the air conditioning compressor and in part due the lack of a convenient access to outside air. However, it is desirable to provide fresh air mixing for all zones as an enhancement to passenger comfort, if it can be done without placing an excessive load on the air conditioning compressor.